(Sorry for any dupes, sent from wrong address originally)
From a developer point of view, I would suggest that a feature should aim to be
as clear to understand with as little “magic" as possible.
If the goal of an immutable class is to allow public properties to be made
read-only, my expectation would be that:
- write access to any public property from outside class context, is an error.
This seems to be pretty much accepted by everyone
- clone still works as expected
There has been some suggestion that clone $immutableObj should not be allowed.
Unless there is some specific language/engine gain by that, what is the point
of having this behaviour?
Existing built-in immutable classes (like DateTimeImmutable) do not prevent
cloning, so why should this?
- regular cloning from within class method(s) is the suggested way to provide
“create a copy of the object with a new value” functionality.
This example was given before, effectively:
public function withValue($val) {
$clone = clone $this;
$clone->val = $val;
return $clone;
}
> On 7 Sep 2016, at 13:57, Michał Brzuchalski <mic...@brzuchalski.com> wrote:
>
> 06.09.2016 9:13 PM "Fleshgrinder" <p...@fleshgrinder.com> napisał(a):
>>
>> I understand the concerns of all of you very well and it's nice to see a
>> discussion around this topic. Fun fact, we are not the only ones with
>> these issues: https://github.com/dotnet/roslyn/issues/159
>>
>> On 9/6/2016 6:01 PM, Larry Garfield wrote:
>>> How big of a need is it to allow returning $this instead of $clone,
>>> and/or can that be internalized somehow as well? With copy-on-write,
>>> is that really an issue beyond a micro-optimization?
>>
>> I asked the same question before because I am also unable to answer this
>> question regarding the engine.
>>
>> However, for me it is more than micro-optimization, it is about identity.
>>
>> final class Immutable {
>> // ... the usual ...
>> public function withValue($value) {
>> $clone = clone $this;
>> $clone->value = $value;
>> return $clone;
>> }
>> }
>>
>> $red = new Immutable('red');
>> $still_red = $red->withValue('red');
>>
>> var_dump($red === $still_red); // bool(false)
>>
>> This is a problem in terms of value objects and PHP still does not allow
>> us operator overloading. A circumstance that I definitely want to
>> address in the near future.
>>
>> But the keyword copy-on-write leads me to yet another proposal, actually
>> your input led me to two new proposals.
>>
>> # Copy-on-Write (CoW)
>> Why are we even bothering on finding ways on making it hard for
>> developers while the solution to our problem is directly in front of us:
>> PHP Strings!
>>
>
> AFAIK CoW in case of objects would be impossible to implement.
>
>> Every place in a PHP program refers to the same string if that string is
>> the same string. In the second someone mutates that string in any way
>> she gets her own mutated reference to that string.
>>
>> That's exactly how we could deal with immutable objects. Developers do
>> not need to take care of anything, they just write totally normal
>> objects and the engine takes care of everything.
>>
>> This approach also has the advantage that the return value of any method
>> is (as always) up to the developers.
>>
>> (Cloning is disabled and results in an error as is because it makes no
>> sense at all.)
>>
>> # Identity
>> This directly leads to the second part of my thoughts and I already
>> touched that topic: identity. If we have two strings their binary
>> representation is always the same:
>>
>> var_dump('string' === 'string'); // bool(true)
>>
>> This is the exact behavior one wants for value objects too. Hence,
>> immutable objects should have this behavior since they identify
>> themselves by their values and not through instances. If I create two
>> instances of Money with the amount 10 and the Currency EUR then they are
>> always the same, no matter what. This would also mean that no developer
>> ever needs to check if the new value is the same as the existing one,
>> nor does anyone ever has to implement the flyweight pattern for
>> immutable objects.
>>
>> A last very important attribute is that it does not matter in which
>> thread an immutable value object is created because it always has the
>> same identity regardless of it.
>>
>> This could easily be achieved by overwriting the object hashes
>> (spl_object_hash) with something that hashes based on the values, and
>> predictably across threads (UUIDs?).
>>
>> # Full Example
>> <?php
>>
>> final immutable class ValueObject {
>>
>> public $value;
>>
>> public function __construct($value) {
>> $this->value = $value;
>> }
>>
>> public function withValue($value) {
>> $this->value = $value;
>> }
>>
>> }
>>
>> class A {
>>
>> public $vo;
>>
>> public function __construct(ValueObject $vo) {
>> $this->vo = $vo;
>> }
>>
>> }
>>
>> class B {
>>
>> public $vo;
>>
>> public function __construct(ValueObject $vo) {
>> $this->vo = $vo;
>> }
>>
>> }
>>
>> $vo = new ValueObject(1);
>>
>> $a = new A($vo);
>> $b = new B($vo);
>>
>> var_dump($a->vo === $b->vo); // bool(true)
>>
>> $a->vo->withValue(2);
>>
>> var_dump($a->vo === $b->vo); // bool(false)
>>
>> $a->vo->withValue(1);
>>
>> var_dump($a->vo === $b->vo); // bool(true)
>>
>> // :)
>>
>> ?>
>>
>> --
>> Richard "Fleshgrinder" Fussenegger
>>
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